Device for the purification of exhaust gases consisting of fluorine-containing compounds in a combustion reactor

ABSTRACT

In a device for the purification of exhaust gases, especially exhaust gases consisting of fluorine-containing compounds, bore holes ( 7,8 ) are made in a ring burner ( 5 ) for the separate supply of combustion gas and oxygen on a single hole circle around a central exhaust gas supply ( 9 ). The ring burner is arranged in a combustion chamber ( 1 ). The combustion chamber is closed up to an annular gap ( 20 ) between a cylindrical jacket ( 4 ) and a front face ( 16 ). Additional oxygen or additional air is introduced into the annular gap ( 20 ).

The present invention relates to a device for the purification ofpollutant-laden exhaust gases in a combustion chamber bythermal-chemical conversion.

In technical plants, especially in vapour phase deposition units andfacilities for the removal of material by means of plasma processes usedin semiconductor plants, pollutant-laden exhaust gases are produced. Animportant group of these exhaust gases contain fluorine-containinghydrocarbons or other fluorine compounds. In addition to thesepollutants, the exhaust gas mainly contains nitrogen as carrier gas.Said pollutants and their reaction products are toxic andenvironmentally harmful, and the exhaust gas must therefore be purifiedin a suitable device in order to remove them. In such devices for thepurification of exhaust gases, the pollutants contained in the exhaustgas are thermally-chemically converted in a combustion chamber in whichthey are subjected to a flame produced by the combustion of a fuel gasin pure oxygen or in air (U.S. Pat. No. 5,183,646). Harmful secondaryproducts of this conversion process (e.g. HF) are subsequently removedfrom the exhaust gas treated in the combustion chamber by means ofsorption or scrubbing processes.

Devices for the purification of exhaust gases normally use a multistageprocess. Sub-processes take place, such as thermal-chemicaldecomposition, oxidation, cooling, sorption, hydrolysis and the removalof liquid and solid reaction products by means of scrubbing. To thisend, the exhaust gas is first introduced in a combustion chamber andsubsequently passes through at least one more process device which e.g.works according to the scrubbing principle (EP 89 110 875, DE 43 20447).

A device for the purification of exhaust gases must fulfil a number ofrequirements: The purification process must be highly efficient, i.e.the amount of primary pollutants contained in the purified exhaust gasmust be as small as possible. In addition, an effective removal ofsecondary pollutants must be achieved in the scrubbing unit. The exhaustgas purification process must further be economical; in particular, theconsumption of fuel gas must be low compared to the volume of theexhaust gas stream to be cleaned. Finally it must be ensured that notoxic carbon monoxide and, above all, no nitrogen oxide is producedduring the purification process.

The design of the combustion chamber and, above all, of the burnerdecisively influences the efficiency and economy as well as thenon-production of secondary pollutant gases.

The combustion chamber is generally designed as a cylindrical body andthe burner, typically a ring burner, is inserted in one of the end facesthereof. The exhaust gas and a fuel gas mixture are supplied to saidring burner, the first one usually via a central inlet and the secondmost simply via an annular gap. Once the fuel gas has been ignited bymeans of an ignition device, a flame forms above the annular gap, intowhich flame the exhaust gas is introduced.

In the fuel gas flame, several reactions take place under the effect ofoxygen (O₂) supplied simultaneously, the most important of which is thecombustion of the fuel gas, e.g. propane (C₃H₈), methane (CH₄), hydrogen(H₂) or mixtures of the aforesaid gases, in order to thermally activatethe pollutant gases and chemically convert the pollutant gases (e.g.CF₄, C₂F₆, CHF₃) into hydrolyzable and adsorbable harmful compounds(e.g. HF) and harmless compounds (e.g. CO₂). Due to the reactionkinetics, it cannot be expected that the desired conversion of thepollutant gases will be complete. This is not even the case when thefuel gas and oxygen are supplied at a stoichiometric ratio (e.g. CH₄ andO₂ at a ratio of 1:2 or C₃H₈ and O₂ at a ratio of 1:5). The so-calledλ-value of the aforesaid gas mixtures is 1 (the air ratio λ is the ratioof the amount of oxygen supplied to the combustion process to the amountrequired for complete combustion). The high content of inert gas (N₂) inthe exhaust gas adversely affects the reaction kinetics, resulting in areduced conversion of the pollutant gas.

As a result, the efficiency of the purification process is poor, i.e.the amount of pollutants remaining in the purified exhaust gas is toohigh.

If the ratio of fuel gas to oxygen is changed so that the amount of fuelgas is higher than at the stoichiometric ratio (λ-value<1), the degreeof pollutant conversion is improved and the formation of nitrogen oxidereduced, but at the same time harmful carbon monoxide and unburned fuelgas will be discharged from the gas purification device. On the otherhand, an increase of the amount of oxygen contained in the fuelgas/oxygen mixture compared to the stoichiometric ratio (λ-value of thesupplied mixture>1) will critically worsen pollutant conversion,particularly in the case of fluorine-containing exhaust gases, resultingin unacceptably high remaining pollutant contents in the purifiedexhaust gas. In addition, harmful nitrogen oxides will be produced in anoxygen-rich, hot flame.

In the purification of pollutant-laden exhaust gases, particularly thosecontaining fluorine-containing compounds, by thermal-chemical conversionin a fuel gas flame, progress has been made by using a burner providedwith a central exhaust gas inlet, in which burner the fuel gas/oxygenmixture is supplied via two concentric annular gaps or via drill holesarranged on two concentric hole circles (EP 0 735 321 A2). If two fuelgas/oxygen mixtures of different composition are supplied in a spatiallyseparate manner, two flame portions having a different thermal-chemicaleffect are realized. A flame portion having a reducing effect isobtained above the inner annular gap or the inner drill holes by meansof an excess of fuel gas (λ<1) compared to the stoichiometric ratio tooxygen, while a flame portion having an oxidizing effect is obtainedabove the outer annular gap or the outer drill holes by means of anoxygen excess (λ>1) (EP 0735 321 A2). Due to the higher concentration ofreducing reactants, such as H atoms and CH_(x) radicals, an increasedamount of pollutant molecules are decomposed in the flame portion withO₂ deficit. Here, the supplied fuel gas is not consumed completely. Thecomplete combustion of the fuel gas and the CO conversion of CO producedin the reducing flame portion into CO₂ take place in a second flameportion with O₂ excess surrounding the first flame portion.

However, even if a burner producing a reducing inner flame portion andan oxidizing outer flame portion due to the spatially separate supply ofdifferent gas mixtures is used for gas purification, there arerestrictions with regard to technical application. For example, thetemperatures required for the thermal reaction under reducing conditionsare only achieved at a certain distance from the burner and, inparticular, only in a restricted volume. On the other hand, oxygen mustbe supplied into the oxidizing flame portion to such an extent that thecomplete oxidation of the fuel gas and of secondary pollutants, e.g. CO,is ensured. As a result, the flame envelope considerably contributes tothe limitation of the volume of the reducing flame portion. Therefore,gas purification devices having a burner as described above and in whicha reducing and an oxidizing fuel gas/oxygen mixture are suppliedseparately are suitable for relatively small exhaust gas volumes only.

In order to be able to purify large exhaust gas volumes effectively,burners have therefore been proposed in which two reducing flameportions are realized by means of two concentric rings or drill holesarranged on two concentric hole circles. Both flame portions areoperated with fuel gas/oxygen mixtures whose λ-values are <1 in order toincrease the volume in which there are favourable conditions forpollutant conversion (EP 011 208 41). In this solution, the completeoxidation of unburned fuel gas and of carbon monoxide produced in thereducing flame portions is achieved by the additional, separate supplyof oxygen or air. Said additional oxygen or additional air is introducedvia nozzles or slots arranged around or near the burner. In this way, afuel gas/oxygen mixture characterized by a λ-value>1 acts in theenvelope of the reducing flame portions. As a result, the flame envelopeconstitutes an additional oxidizing flame portion (EP 011 208 41).

The aforesaid solution still has the drawback that the reducing flameportion, which is advantageous for pollutant conversion, remains limiteddue to the fact that the flame's circumference is exposed to oxygenacting thereon. Moreover, the additional oxygen inlet near the burnerleads to such high temperatures in a part of the flame that harmfulnitrogen oxides will be produced there.

Burners using fuel gas/oxygen mixtures are also problematic in that theburner's design must be precisely adapted to match the specificoperating conditions (type of pollutant gas, amount of exhaust gas andresulting amount of mixture). If the proportion of fuel gas to oxygen ischanged, the discharge velocity from the burner and consequently theflame velocity will change. However, a discharge velocity of the fuelgas mixture higher than the flame velocity entails the risk that theflame will be extinguished. If, on the other hand, the dischargevelocity of the mixture is lower than the flame velocity, backfires mayresult. Both risks must be eliminated by appropriately adapting theburner. However, a gas purification device must be able to operatesafely without any need to adapt the burner to differing types ofpollutant gas or amounts of exhaust gas; the need to adapt the burner isa disadvantage.

If the fuel gas and oxygen are supplied separately to the burner, theaforesaid operating conditions, i.e. the type of pollutant gas andamount of exhaust gas, can vary widely during operation without any needto change the burner's design. Adaptations are made by appropriatelycontrolling the amounts of fuel gas and oxygen supplied. Exhaust gastreatment in reducing and oxidizing flame portions is possible even ifthe fuel gas and oxygen are supplied separately to the burner. Asuitable burner is provided with two concentric slots or drill holesarranged on two concentric hole circles. Fuel gas and oxygen areintroduced separately, at a ratio corresponding to a λ-value of <1 oncethey have mixed above the burner, and a reducing flame will form abovethe burner. If additional oxygen or air from the surroundings of theburner is supplied into the flame envelope, an oxidizing flame portionis obtained in which remaining fuel gas will be converted and COproduced in the reducing flame portion will be oxidized to CO₂.

The aforesaid design and operation mode of the burner still have thedrawbacks that the reducing flame portion remains limited, resulting ina limited efficiency of the pollutant gas conversion process and theproduction of nitrogen oxide in hot, oxidizing flame portions. Anotherdisadvantage is the formation of soot in those parts where the localλ-value of the immediately surrounding gas mixture is very low, i.e.near 1.

The object of the present invention is to eliminate drawbacks ofstate-of-the-art devices used for the purification of exhaust gases bythermal-chemical conversion. It must be ensured that large exhaust gasvolumes are cleaned of pollutants with high efficiency and good economyas regards fuel gas consumption. No soot must deposit on the burner. Thepurified exhaust gas must have very low contents of unburned fuel gas,carbon monoxide and, in particular, nitrogen oxide.

The inventive solution is based on the assumption that the purificationof pollutant-laden exhaust gases, particularly those containingfluorocarbon compounds and other fluorine-containing compounds andnitrogen as carrier gas, by thermal-chemical conversion takes place in acylindrical combustion chamber which is provided with a burner andintegrated with a subsequent scrubbing unit. The burner has a centralexhaust gas inlet. The fuel gas and oxygen are supplied separately tothe burner, up to its discharge nozzles for forming a flame. The hot,treated exhaust gas exiting from the combustion chamber is aftertreatedwith a scrubbing agent in a scrubbing unit. During aftertreatment, thehot exhaust gas is cooled and harmful secondary products are removedfrom the exhaust gas.

According to the invention, the burner is a ring burner provided withdrill holes for the separate supply of fuel gas and oxygen, which holesare arranged on a single hole circle around the central exhaust gasinlet. Said separate supply is achieved by introducing fuel gas andoxygen in a locally alternate manner via adjacent drill holes. To thisend, two annular channels are arranged in the ring burner, which arealternately connected to the drill holes on the hole circle. Aconnecting pipe leads out of the burner from each annular channel. Fuelgas is supplied to the burner through one of the connecting pipes,oxygen is supplied through the other.

The alternate supply of fuel gas and oxygen via the drill holes ensuresthat both gases will mix thoroughly at the very moment they exit fromthe burner. The flame forms very close to the surface of the burner. Inthis way, only hot flame portions come into contact with the exitingpollutant gas. Pollutant gas is prevented from just mixing with parts ofthe fuel gas. This obviously minimizes soot deposits on the burner,particularly near the exhaust gas inlet.

The aforesaid burner is particularly suitable for burning fuel gassupplied separately from oxygen with an oxygen deficit corresponding toλ-values down to 0.6 in the fuel gas/oxygen mixture which is presentdirectly above the burner. The ring burner ensures the formation of astable, homogenous flame with effective energy output for the mostdifferent fuel gas and oxygen streams, thus enabling adaptation todiffering pollutant gases and exhaust gas streams. Said adaptation doesnot require the exchange of the burner for another one having adapteddrill holes. In addition to fluorocarbons and other fluorine compounds,the burner is suitable for removing reactive pollutants such as SiH₄,WF₆ and TEOS.

According to the invention, said burner is used in a combustion chamberwhich is closed except for an annular gap between the cylindrical jacketof the combustion chamber and the end face of the combustion chamberlocated opposite the burner. In this way, no air, and consequently nooxygen, flows to the vicinity of the burner, e.g. via slots in the wallof the combustion chamber. The combustion chamber is at least tightenough that air flowing in from the outside accounts for less thanapprox. 3% of the oxygen supplied into the burner.

If fuel gas and oxygen are supplied separately to the burner and theburner is operated with a fuel gas excess corresponding to a mixturewhose λ-value is e.g. 0.8, a reducing flame portion will not just formabove the burner, but the whole flame will have a reducing effect duringthe thermal-chemical conversion of the pollutants. Since no air flows infrom outside the burner, i.e. the flame is not exposed to oxygen,reducing conditions are obtained even in the flame envelope. Finally,there will be reducing conditions virtually throughout the whole volumeof the combustion chamber. Compared to the volume of a flame havingreducing and oxidizing flame portions, a larger volume with a highconcentration of H atoms and CH₃ radicals, i.e. the reactants requiredfor reducing the pollutants (e.g. C₂F₆, CF₄, CHF₃) to other gaseousreaction products such as HF, is achieved. The energy needed for thethermal activation of the reactants is produced by burning parts of thefuel gas, i.e. the oxidation process going on in parallel. Owing to theaforesaid conditions, the device according to the invention can beoperated at a lower temperature (T<≈1,200°) than a device havingreducing and oxidizing flame portions. The higher concentration of thereducing reactants compensates, or even more than compensates, for thepositive influence a higher temperature would have on the reactions. Inthis way, the pollutants are converted in a highly efficient manner at arelatively low temperature. The conditions for the formation of nitrogenoxides from the neutral gas portion of the exhaust gas are stronglylimited since the thermal-chemical conversion of the pollutants takesplace at a relatively low temperature and with an oxygen deficit in thereaction chamber. As a result, the amount of nitrogen oxide contained inthe treated exhaust gas stream exiting from the combustion chamber isstrongly reduced.

In the aforedescribed mode of operation, the hot exhaust gas streamexiting from the cylindrical jacket of the combustion chamber does notonly contain the reaction products of the combustion process and thethermal-chemical conversion, i.e. mainly CO₂, HF, CO and H₂O, but alsounburned fuel gas constituents (CH₄ and CO) due to the oxygen deficit inthe combustion chamber. The hot exhaust gas is not completely oxidizedat the end of the combustion chamber.

For complete combustion, the exhaust gas stream is subjected to anotheroxidation process. To this end, in a first embodiment of the inventiontwo or more pipes are arranged in said annular gap between thecylindrical jacket of the combustion chamber and the end face of thecombustion chamber located opposite the burner, which pipes aredistributed on the gap's circumference, face towards the axis of thecombustion chamber and serve to supply additional oxygen or air. Inanother embodiment, additional oxygen or air is supplied into saidannular gap evenly along its circumference via an annular channelarranged at the end face of the cylindrical jacket of the combustionchamber. Moreover, a body, preferably a plate, made of a heat-resistantmaterial is arranged in front of the metallic end face of the combustionchamber in a heat-insulated manner, e.g. by means of retaining webs. Thehot exhaust gas stream hits this plate. In order to be passed on to thesubsequent scrubbing unit via the annular gap, it is deflected by 90°and distributed radially. Said deflection and radial distribution resultin turbulence in the hot exhaust gas stream. The aforesaid plateprevents the hot exhaust gas stream from contacting the end face of thecombustion chamber. The end face is relatively cold since it alsodelimits the scrubbing unit and is therefore cooled by the scrubbingliquid (T ranging between 20° and 90° Celsius). In contrast, the platearranged in a heat-insulated manner almost heats up to the temperatureof the hot exhaust gas stream, i.e. T>800° Celsius, on its side facingtowards the combustion chamber. While the hot exhaust gas stream flowsout of the gap, oxygen (or air) enters via the pipes arranged in thegap, enhancing the turbulence in the hot exhaust gas stream in the areaof contact with the latter. The amount of oxygen (or air) let in is suchthat a λ-value of >1, preferably λ=1.2, is achieved in the mixture ofsupplied oxygen (or supplied air) and hot exhaust gas. This means,oxygen (or air) is supplied to a degree that the oxygen deficit in thecombustion chamber is at least neutralized. With the aforesaid λ-valueof the mixture of hot exhaust gas and supplied oxygen (or supplied air),oxidizing conditions are achieved at the prevailing temperature(800°<T<1,200°). The reactions which preferably take place here have thecharacter of a secondary combustion. CO which has been produced in thecombustion chamber in the primary combustion process and remaining,unburned fuel gas are converted into CO₂ and H₂O. The temperature in thearea between the end of the cylindrical jacket and said hot plate islower than required for the conversion of the fluorine-containingpollutants, but high enough for the combustion of CO and remaining fuelgas.

The strong turbulence produced and the resulting thorough mixing, thetemperature level of the exhaust gas stream in this area and theadapted, high λ-value ensure a complete, effective afterburning in arather limited space in the transition area to the scrubbing unit.

Once complete combustion has finished, the exhaust gas enters thescrubbing unit. Here, the hot exhaust gas stream is cooled, HF isneutralized and solid particles formed during the combustion process arewashed out. The purified, cooled exhaust gas is then passed on into theexhaust air channel of the production plant.

The use of the device according to the invention is advantageous in thatit ensures a reduction of the specific amount of fuel gas required, thusimproving the economy of the exhaust gas purification process. Thereduction of the amount of fuel gas required is achieved by the reducedtotal flow due to the fact that air or O₂ are prevented from reachingthe vicinity of the burner. The special solutions described above ensurea highly efficient pollutant conversion process. At the same time, thesolution in respect of the burner ensures the formation of a stableflame for widely differing fuel gas/oxygen ratios, differing pollutantgases in the exhaust gas and differing amounts of pollutant gas withoutany need to adapt the design of the burner.

A particularly significant advantage is the strong reduction of theamount of nitrogen oxides discharged with the purified exhaust gas. Saiddischarge is approximately five times lower than in a state-of-the-artdevice using a ring burner, supply of a fuel gas/oxygen mixture andsupply of additional oxygen in the area of the ring burner, providedboth devices are operated under similar conditions.

Another advantage of the device according to the invention is that itcan also be used for the purification of exhaust gases containingpollutants whose thermal-chemical conversion requires a relatively lowenergy input, e.g. SiH₄, without changing its design.

As regards the design of the ring burner, the individual holes areconveniently drilled with uniform diameter on a single hole circlearound the central exhaust gas inlet and distributed evenly on thecircumference of said circle. During the alternate supply of fuel gasand oxygen into said drill holes, a thorough mixing of fuel gas andoxygen is ensured by the fact that the gas exiting from a hole is indirect contact with the gases exiting from the two adjacent holes.

However, it can also be useful that the drill holes which aredistributed evenly on the hole circle around the central exhaust gasinlet have two different diameters. If e.g. the burner is operated withmethane and oxygen and reducing conditions are to be achieved in thecombustion chamber, corresponding to a λ-value of 0.8 of the gasesentering the chamber and mixing directly above the burner surface (fuelgas and oxygen), the required methane gas stream is approx. 0.6 timesthat of oxygen. If the drill holes' areas differ by the factor indicatedbefore, it will be possible to adapt the discharge velocities of the twogases in the drill holes of the burner to one another. In this way, thestability of the flame can be improved, for example.

In the aforedescribed embodiment of the ring burner, the alternate,separate supply of fuel gas and oxygen into adjacent drill holes on theburner surface is achieved by arranging two annular channels inside theburner and alternately connecting the drill holes in the burner surfaceto one of these annular channels. The burner is provided with a fuel gassupply pipe for one of these annular channels and an oxygen supply pipefor the other annular channel.

As regards said body in front of the end face of the combustion chamber,it can also be useful that this body be dome-shaped with a vault depthof 15 mm to 60 mm, preferably 20 mm, and a diameter larger than that ofthe combustion chamber, but smaller than the diameter of the end face,which dome-shaped body is made of heat-resistant, corrosion-proof steeland is arranged in a heat-insulated manner with its concave side facingtowards the ring burner. In this way, the hot exhaust gas exiting fromthe combustion chamber is prevented from being held up in front of thisbody, which may adversely affect the mixing of hot exhaust gas exitingfrom the combustion chamber and additional oxygen blown in.

If pipes are arranged for the additional supply of oxygen (or air) intosaid gap at the end of the combustion chamber, the axes of these pipesare conveniently inclined relative to the axis of the combustion chamberat an angle of 60° to 85°, preferably 80°. This ensures that scrubbingagent from the scrubbing unit will not enter the combustion chamber, butflow outwards along the inclined pipes.

In addition, said pipes conveniently extend approx. 15 mm to 50 mm,preferably 25 mm, beyond the edge of the end face of the combustionchamber into the gap between the cylindrical jacket of the combustionchamber and the end face, but not beyond the edge of the cylindricaljacket. This measure ensures that no scrubbing agent enters the pipesfor the additional supply of oxygen (or air) and the ends of said pipeswill not heat up too much and, in consequence, corrode.

If an annular channel is arranged for the additional supply of oxygen(or air) into said annular gap, the annular channel is convenientlyrealized by means of a cylindrical pipe arranged concentrically to thecombustion chamber and the supply pipe for oxygen (or air) is arrangedat the opposite end of said annular channel. In this way, a more evendistribution along the channel's circumference is achieved. In addition,the annular channel conveniently has a width of 1.5 to 2 mm.

The invention will now be explained by means of an exemplary embodimentof the device and with reference to the drawings, FIG. 1 to FIG. 4, inwhich:

FIG. 1 shows a schematic longitudinal section of a device for thepurification of exhaust gas in which additional oxygen is supplied viapipes;

FIG. 2 shows a schematic longitudinal section of a device for thepurification of exhaust gas in which additional oxygen is supplied viaan annular channel;

FIG. 3 shows the schematic plan view of a ring burner;

FIG. 4 shows the schematic cross section of a ring burner.

The device essentially consists of a cylindrical combustion chamber (1)which is made of a corrosion-proof material and arranged in a housing(2) made of corrosion-proof steel. The combustion chamber has a diameterof 100 mm and a height of 400 mm. In the area of the base plate (3) andthe cylindrical jacket (4), the combustion chamber is closed so that nooutside air can flow in. The ring burner (5) having an outer diameter of50 mm is arranged centrally in the base plate (3). The ring burner (5)is provided with a central drill hole (6) whose diameter is 12 mm andwith the connection (9) for supplying the exhaust gas into thecombustion chamber (1). In the surface of the ring burner (5), drillholes (7) with a diameter of 1 mm and drill holes (8) with a diameter of1.2 mm for the separate supply of fuel gas and oxygen into the ringburner (5) are alternately arranged at even distances from one anotheron a hole circle (28) whose diameter is 30 mm. The ring burner issupplied with fuel gas via the connection (10) and with oxygen via theconnection (11). Inside the ring burner (5), the supplied fuel gas isdistributed along the annular channel (29) and the supplied oxygen isdistributed along the annular channel (30). The drill holes (7) areconnected to the annular channel (29) and the drill holes (8) areconnected to the annular channel (30).

In order to operate the combustion chamber, 20 slm (standard litres perminute) CH₄ and 32 slm oxygen are supplied into the ring burner. Themixture of CH₄ and oxygen resulting directly above the burnercorresponds to a λ-value of 0.8, i.e. it is a hypostoichiometric mixtureof oxygen and fuel gas. Upon actuation of the ignition device (12), anannular flame (13) forms above the ring burner (5), close to the surfacethereof, which flame changes into a flame (14) having a homogeneouscross section at a greater distance from the surface of the ring burner(5). The supply of oxygen and fuel gas is controlled with the aid ofsensor signals of the monitor (15) facing towards the flame (14). 160slm exhaust gas is introduced in this flame via the connection (9) andthe central drill hole (6), which exhaust gas essentially consists of158 slm nitrogen and approx. 2 slm C₂F₆.

Due to the λ-value of the mixture of separately supplied oxygen andseparately supplied CH₄, the flame (14) has a reducing effect over itsentire cross section once it has fully developed. Said reducing effecton the pollutant gas is also achieved in the flame envelope and evenbeyond, virtually throughout the whole volume (17) of the combustionchamber (1), since the flame is not exposed to air, i.e. oxygen, actingon it from the outside in the area of the ring burner (5). Essentially,CH₄, O₂ and C₂F₆ are converted into HF, CO₂, CO and H₂O in the flame(13, 14) and in the remaining volume of the combustion chamber. Theproduction of nitrogen oxides is largely avoided. A mixture of hot,treated exhaust gas (consisting of N₂, HF, CO₂ and CO) and unburned CH₄whose temperature ranges between 800° and 1,200° Celsius flows out ofthe cylindrical part of the combustion chamber (1) towards the hot body(19), as indicated by the arrows (18).

The hot body (19) is made of corrosion-proof steel and has a diameter of260 mm and a thickness of 2 mm. It is dome-shaped with a vault depth of20 mm and is arranged in front of the cooled end face (16) having adiameter of 300 mm in a heat-insulated manner and with its concave sidefacing towards the ring burner (5). In the device according to theexemplary embodiment, the cooled end face (16) is also dome-shaped witha vault depth of 40 mm. The convex side of the cooled end face (16)faces towards the scrubbing unit (25). In one embodiment (FIG. 1), threepipes (21) which are arranged at an angle of 120° relative to oneanother in front of the body (19) and have an outer diameter of 6 mmextend 25 mm beyond the edge of the end face (16) into the 60 mm widegap (20) between the cylindrical jacket (4) and the end face (16) of thecombustion chamber (1), but not beyond the edge of the cylindricaljacket (4) into the combustion chamber. Said pipes are inclined at anangle of 80° relative to the axis of the combustion chamber (1). Thepipes (21) are connected with one another via an annular pipe (22) whichis arranged outside the housing (2) and to which additional oxygen issupplied via the connection (23). 11 slm oxygen are blown in via thepipes (21) at a rate of approx. 60 m s⁻¹.

In another embodiment (FIG. 2), a double pipe (33) is arrangedconcentrically to the cylindrical combustion chamber (1) such that anannular channel (32) is formed between the cylindrical combustionchamber and the double pipe. Additional oxygen is supplied via thisannular channel into the annular gap (20) evenly along the circumferencethereof. Said oxygen (or air) is supplied to the annular channel via aconnection (34) arranged at the opposite end of the annular channel. 20slm oxygen (or 100 slm air) is supplied into the annular channel (32).

The hot exhaust gas stream and the oxygen blown in mix with one anotherdue to turbulence in the area of the gap (20) in front of the body (19).Here, a hyperstoichiometric ratio of oxygen to unburned fuel gas resultsin the mixture of additional oxygen supplied and unburned CH₄. Theλ-value of this gas mixture would be higher than 1.2. Therefore, theturbulent mixture of hot exhaust gas and additional oxygen supplied hasan oxidizing effect. In a secondary combustion process, unburnedconstituents of the exhaust gas are completely burned; in particular, COwhich has been produced in the reducing volume of the combustion chamber(1) is converted into CO₂ and H₂O.

The treated exhaust gas then flows out of the gap (20) towards thescrubbing unit (25), as indicated by the arrows (24). The scrubbingliquid (27) is supplied to said unit via the connection (26). Saidliquid cools the hot exhaust gas to below 50° C. Hydrogen fluoride (HF)contained in the cooled exhaust gas is hydrolyzed and neutralized in thescrubbing liquid (a 1% sodium hydroxide solution).

The cleaned exhaust gas is discharged to the surrounding atmosphere, asindicated by the arrows (31), either directly via an exhauster or viathe central exhaust air unit of the semiconductor plant.

The carbon monoxide content of the cleaned exhaust gas is 10 ppm. Thedischarge of nitrogen oxides is strongly reduced; in the exemplaryexhaust gas purification process it is as low as ≈0.1 mol m⁻³.

REFERENCE NUMERALS

-   1. Combustion chamber-   2. Housing-   3. Base plate-   4. Cylindrical jacket-   5. Ring burner-   6. Central drill hole-   7. Drill holes in the burner-   8. Drill holes in the burner-   9. Connection for exhaust gas-   10. Connection for fuel gas-   11. Connection for oxygen-   12. Ignition device-   13. Annular flame-   14. Homogeneous flame-   15. Monitor-   16. Cooled end face-   17. Volume of the combustion chamber-   18. Arrows-   19. Body-   20. Gap-   21. Pipes for additional oxygen-   22. Annular pipe-   23. Connection for oxygen-   24. Arrows-   25. Scrubbing unit-   26. Connection for scrubbing liquid-   27. Scrubbing liquid-   28. Hole circle-   29. Annular channel for fuel gas-   30. Annular channel for oxygen-   31. Arrows-   32. Annular channel for additional oxygen-   33 Cylindrical double pipe-   34. Connection for oxygen

1. A device for the purification of exhaust gases containing nitrogenand pollutants, particularly those containing fluorocarbon compounds orother fluorine compounds, by thermal-chemical conversion in acylindrical combustion chamber integrated with a subsequent scrubbingunit and using a burner with central exhaust gas supply and separatesupply of fuel gas and oxygen characterized in that the ring burner isprovided with drill holes which are arranged on a single hole circle andensure the locally alternate, simultaneous supply of fuel gas andoxygen, the combustion chamber is closed except for an annular gapbetween the cylindrical jacket and the end face of the combustionchamber located opposite the burner, that a body, preferably a plate,made of heat-resistant and heat-insulating material is arranged in frontof said end face of the combustion chamber, and that a double pipe isarranged concentrically to the cylindrical jacket of the combustionchamber, thus forming an annular channel for the supply of additionaloxygen or additional air.
 2. A device according to claim 1,characterized in that the annular channel between the cylindrical jacketof the combustion chamber and the double pipe for the supply ofadditional oxygen or additional air arranged concentrically thereto hasa radial width of 1.5 to 2 mm.
 3. A device according to claim 1,characterized in that the annular channel opens into the gap between thecylindrical jacket of the combustion chamber and the end face parallelto the direction of the axis of the combustion chamber.
 4. A deviceaccording to claim 1, characterized in that the annular channel opensinto the gap between the cylindrical jacket of the combustion chamberand the end face radially at an angle of 90° relative to the axis of thecombustion chamber and facing towards said axis.
 5. A device accordingto claim 1, characterized in that the annular channel opens into the gapbetween the cylindrical jacket of the combustion chamber and the endface radially at an angle of 90° relative to the axis of the combustionchamber and facing away from said axis.
 6. A device according to claim1, characterized in that the drill holes in the surface of the ringburner are distributed evenly on a hole circle and all have the samediameter.
 7. A device according to claim 1, characterized in that thedrill holes in the surface of the ring burner are distributed evenly ona hole circle and have two different diameters alternating with oneanother.
 8. A device according to claim 1, characterized in that twoannular channels are provided inside the ring burner, that the drillholes in the ring burner are alternately connected to one of theseannular channels, and that the ring burner is provided with a connectionfor the supply of fuel gas to one of these annular channels and with aconnection for the supply of oxygen to the other annular channel.
 9. Adevice according to claim 1, characterized in that a body, preferably aplate, which is made of heat-resistant material and whose diameter islarger than that of the combustion chamber, but smaller than that of theend face (16) is arranged in front of said end face of the combustionchamber in the area of the gap in a heat-insulating manner.
 10. A deviceaccording to claim 1, characterized in that a dome-shaped body with avault depth of 15 mm to 60 mm, preferably 20 mm, which is made ofheat-resistant, corrosion-proof steel and whose diameter is larger thanthat of the combustion chamber is arranged in front of said end face ofthe combustion chamber in the area of the gap in a heat-insulatingmanner and with its concave side facing towards the ring burner (5).